Fibers for spunlaced products

ABSTRACT

Fractionated cellulosic fibers having a length weighted average fiber length of at least 2.4 mm and less than 2% by weight of fines. In another embodiment the fractionated cellulosic fibers have a length weighted average fiber length of at least 3.5 mm. In another embodiment the fractionated cellulosic fibers have a length weighted average fiber length in the range of 2.4 mm to 3.8 mm. The fibers may be used in spunlacing or airlacing.

BACKGROUND OF THE INVENTION

The oldest technique for consolidating a web is mechanical bonding. The fibers are entangled to give strength to the web. One method of entangling the fibers is spunlacing. Spunlacing is also known as hydroentanglement, jet entanglement, water entangled, hydraulically needled or hydraulically loomed. In spunlacing high pressure jets of water strike a web so that the fibers in the web become entangled and form a nonwoven web. The jets are usually perpendicular to the web but may be angled in order to provide different properties to the web.

The web may be formed by airlaying the fibers onto a moving belt to form a mat and passing the mat under a series of high pressure jets that entangle the fibers. The fibers may be blended or two or more webs may be airlaid before the spunlacing step.

There are variations on spunlacing. One variation is airlacing. In airlacing, a mat of fibers is airlaid onto a nonwoven web and the fibers in the air laid mat are hydroentangled with the nonwoven mat. This process is disclosed in U.S. Patent Application Publication U.S. 2002/0168910, published Nov. 14, 2002, and in U.S. Pat. No. 5,009,747, issued Apr. 23, 1991. The process includes fiber preparation, carding, pre-entanglement, air-laying, spunlacing, drying and winding.

Airlacing is generally a combination of carding and air-laying but can also include airlaying blended fibers or two or more airlaid webs. The web is bonded by spunlacing so that the fibers of the various layers are entangled to produce the web.

In the airlacing process, the non-woven web is carried on a conveyor. The cellulosic pulp fibers are deposited as a mat on the web by means of a stream of air. A second web may be placed on the fiber mat, and the entire composite mat may be compacted if desired.

In either spunlacing or airlacing, the main entanglement takes place after final air-laid formation. The mat or composite mat is bonded together by treatment with a series of water jets, which entangles the pulp fibers into the non-woven web. The water jets may act on one or both sides of the mat.

Cellulosic pulp fibers may be used in the formation of spunlaced or airlaced mats A problem is that the fines or short fibers normally found in cellulosic pulp reduce the efficiency of the process. Some of the fines and short fibers are removed by the water jets during the hydroentangling process. Water is reused in the hydroentangling process and the fines and short fibers need to be filtered out of the water stream before it is returned to the water jets.

Another problem is that the fines and short fibers that remain in the mat do not become attached to the non-woven web and may dust from the formed product.

SUMMARY

In the present invention the fibers are separated into one fraction in which the length weighted average fiber length is longer than the length weighted average fiber length of the starting pulp, and another fraction in which the length weighted average fiber length is shorter than the length weighted average fiber length of the starting pulp. These are referred to as the long and short fiber fractions. The majority of the fines remain with the short fiber fraction.

The long fiber fraction is used in a spunlaced process. The long fiber fraction is made into a pulpboard or bale at the pulp mill and transported to the plant where the spunlacing process take place.

The fractionated cellulosic fibers are formed into a hydroentangled mat. In one embodiment the fractionated cellulosic fibers have a length weighted average fiber length of at least 2.4 mm. In another embodiment the fibers have a length weighted average fiber length in the range of 2.4 mm to 3.5 mm. In another embodiment the fibers have a length weighted average fiber length of at least 3.5 mm. The mat of fibers may be entangled with a nonwoven web or sandwiched between and entangled with nonwoven webs.

This use of this long fiber fraction has a number of benefits when compared to using the entire pulp material. The energy needed to fiberize a pulp sheet or bale having only a long fiber fraction is less than the energy required to fiberize a pulp sheet or bale of the prefractionated pulp. There is less loss of fiber in the hydroentangling process when only the long fiber fraction is used giving a more efficient process. The process is also more efficient because with the reduced amount of fines and short fibers being recycled through the process there is less time needed to clean and maintain filters. The airlaced product is improved due to greater adhesion between the long fibers and the non-woven web.

There is an added benefit when the pulp mill has an associated paper mill. The fines and short fibers fraction may be used in the paper furnish in place of highly refined fiber that is normally used in paper furnish.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of apparatus for spunlacing a mat of fibers.

FIG. 2 is a schematic view of apparatus for airlacing fibers into a web.

DETAILED DISCUSSION

FIG. 1 shows an apparatus for spunlacing a mat of fibers. The mat 10 is carried onto a conveyor 12 and passes under a series of water jets 14, 16, 18 and 20. The water jets are usually at increasing water pressure. There is a vacuum chamber 22 underneath the conveyor 12. The water from the jets 14, 16, 18 and 20 pass through the mat 10 entangling the fibers. The water is pulled from the mat by the vacuum in chamber 22. The mat of entangled fibers 10′ is transported into the dryer section 24 and dried. Dryer cans 26 are shown. The mat is then rolled up or boxed for shipment.

FIG. 2 shows airlacing. The apparatus would be the same as the apparatus in FIG. 1. The difference between FIGS. 1 and 2 is the web 30. The water jet 32, which is one of a series of water jets, displaces the fibers in the mat 10″ downwardly and entangles the fibers with the web 30. The composite web is then dried as shown in FIG. 1.

The pulp fibers that may be used are mechanical pulp fibers, thermomechanical pulp fibers, chemithermomechanical pulp fibers, and chemical pulp fibers made from either kraft or soda processes. The pulp fibers can be bleached or unbleached. The pulp fibers can be from hardwoods or softwood or combinations of hardwoods and softwoods. The pulp fibers can be dried or never dried.

In these examples southern pine pulp was used, but any wood pulp can be separated in the same fashion and used. The criteria for pulp selection is based on being able to produce a long fiber fraction in which the length weighted average fiber length is greater than or equal to 2.4 mm. In one embodiment the long fiber fraction has a length weighted average fiber length in the range of 2.5 to 3.8 mm. These fibers would be used in a spunlaced or airlaced process.

The length weighted average fiber length (LWAFL) is determined by the following formula: LWAFL=Σ(l ² _(x) *n _(x))/Σ(l _(x) *n _(x)) where l=bin median length, n=the bin fiber count and x=bin #.

The pulp fibers may be blended with other fibers including other pulp fibers, may be in layers including layers of other pulp fibers or layers of other types of fibers. These fibers may be spunlaced or placed on a nonwoven web or between nonwoven webs and airlaced. The nonwoven webs may made of polyester, polypropylene, rayon, lyocell or aramid fibers.

In one embodiment the pulp fibers in the mat would have a length weighted average fiber length is greater than or equal to 2.4 mm. In another embodiment the pulp fibers in the mat would have a length weighted average fiber length in the range of 2.4 to 3.8 mm. In another embodiment the pulp fibers have a length weighted average fiber length greater than or equal to 3.5 mm. The pulp fibers would have less than 2% by weight fines as defined below.

In the spunlacing or airlacing process the fiber mat would be placed under a series of jets, each of the jets being at a higher pressure than the previous jet. The interlocked fiber mat would then be rolled into rolls for shipment.

In the following examples the fiber lengths and percent fines was determined using a Fiber Quality Analyzer (Optest Equipment Inc., Hawksbury, Ontario, Canada). Fines are determined by the Fiber Quality Analyzer as fibers with a length less than 0.2 mm.

In the following examples the following terminology is used. The starting pulp is an unfractionated commercial Southern pine pulp fiber from the Weyerhaeuser Plymouth, N.C. pulp mill. It is designated PL416. It may be in a dried pulp sheet form PS PL416, a never dried form ND PS416, or a slushed form S PL416. The difference in these forms is the amount of water with the fiber known as the consistency of the fiber. The fiber lengths and the weight percentages of each fiber length will be the same within the usual manufacturing tolerances.

This starting pulp will be fractionated into a long fiber fraction (LF) and a short fiber fraction. The fractionation will be either by a Bauer-McNett fractionator resulting in a Bauer-McNett long fiber fraction BM LF or by a Bird Screen resulting in a Bird Screen long fiber fraction BS LF.

PS PL416 was fractionated using a Bauer-McNett fractionator. The Bauer-McNett fractionator was set up with screens of +12, +28, +48, +100, +200 and −200. The material from the +12 and +28 screens were combined for the long fiber fraction, while the remaining fractions were combined for the short fiber fraction.

ND PL416 was fractionated using a Bird Screen. The Bird Screen used was a Bird Model 100, with a 0.040″ holes in the basket. In the Bird Screen trials the fiber was separated into a 17% short fiber fraction and an 83% long fiber fraction. The ND PL416 was diluted to 1.25% consistency and delivered to the screen at about 40 psi. The accepts flow rate was 100 gpm. This resulted in an accepts stream at 0.25% and a rejects stream at 1.5%.

The long fiber fraction and the PL416 pulp were made into 12″×12″ handsheets with a basis weight of 750 g/m². The fiberization energy was determined using a Kamas hammermill. The long fiber pulp fraction has a lower fiberization energy than the unfractionated fiber.

The results are shown in Table 1. TABLE 1 Percent Fiberization Length Weighted fines, energy, Sample Average Fiber length, mm % kj/kg Bauer-McNett PL416 2.33 4.8 72 Long fiber fraction 3.73 1.1 23 Short fiber fraction 1.01 14.4 Bird Screen PL416 2.29 5.0 123 Long fiber fraction 2.46 2.8 101 short fiber fraction 1.38 13.8

Samples of the BS LF fibers, the ND PL416 fibers from the Bird Screen trials and S PL416 fibers were made into 12″×12″ handsheets with a basis weight of 750 g/m². This approximates the basis weight of commercial pulp board. The handsheets were tested for Mullen burst and Burst index using TAPPI method T403 om-97, and tensile properties using TAPPI method T494 om-96. The data is shown in Table 2. As can be seen in the Table, the long fiber fraction had lower Kamas fiberization energy, mullen, burst, tensile index and tensile energy absorption (TEA). The ND PL416 was nearly the same as the S PL416. TABLE 2 Property S PL416 BS LF ND PL416 Kamas energy, kJ/g 75 65 79 Mullen burst, kPa 1587 1503 1545 Burst index, kN/g 1.87 1.84 1.88 Tensile index, Nm/g 10.62 9.65 10.35 TEA, J/m² 223 189 251

The BS LF fiber was converted to a pulp board on a Noble & Wood pilot paper machine. This BS LF pulp board and PL 416 pulp board were then fiberized using a Fitz hammermill. The pulp fibers were then compared for runnability or through-put on a Dan-Web air laid machine. The pulps were hand feed into both sides of the distribution head of the Dan Web machine. The load on the distribution rotor in the head was monitored, and maintained at a level that represented a feed rate that was about 90% of the maximal feed rate for the system. The forming wire was run at a constant speed. Under these conditions the through-put can be measured by monitoring the basis weight of the web. The results of the trials are given in Table 3. As can be seen, the basis weight of the BS LF pulp was greater than PL416 pulp indicating a higher through-put for the BS LF pulp. TABLE 3 basis weight g/m² energy hz density g/cm² caliper mm PL416 140 0.618 0.0352 4.185 BS LF 187 0.606 0.0393 4.757

The BS LF board and PL416 pulp were used in a test of the applicability of the long fiber fraction for improvements to the airlaced process. The two pulps were airlaid into a web of 100 g/m² using an M&J air laid line. This was layered onto a 30 g/m² carded polyester web. This composite was then run through spunlacing equipment at North Carolina University, Raleigh, N.C. The hydroentangling was done by passing under three water jets with increasing pressure at each jet. Three different pressure regimes were conducted to determine the effect of pressure on the pulp fibers. Generally lower water pressures are used to hydroentangle wood pulp fibers because of the potential loss of fiber. The web was oriented such that the water jets impinged on the wood pulp side of the web.

The three pressure regimes used in the water jet heads were:

-   -   (a) 200 pounds per square inch (psi) in the first head, 400 psi         in the second head and 800 psi in the third head (200/400/800).         This is equivalent to approximately 14/27/54 bars.     -   (b) 200 pounds per square inch (psi) in the first head, 600 psi         in the second head and 1000 psi in the third head         (200/600/1000). This is equivalent to approximately 14/41/68         bars.     -   (c) 200 pounds per square inch (psi) in the first head, 600 psi         in the second head and 1200 psi in the third head         (200/600/1200). This is equivalent to approximately 14/41/82         bars.

The basis weight loss was determined for each of these regimes for both BS LF fiber and PL416 fiber. The results are shown in Table 4. TABLE 4 100/400/800 200/600/1000 200/600/1200 PL416 20% 29% 36% BS LF  8%  5% 15%

The long fiber may be used in hydroentangling as shown.

In a pulp mill unbleached pulp from brownstock system or bleached pulp from the bleaching system or mechanical pulp may be diluted and sent to a screening system. The type and size of screen will depend on the kind or amount of fractionation. A typical screen that may be used is the Kandant Black Clawson model 400 UVC with 590 rpm motor. The size of motor may vary. An appropriate sized pump would be used to carry the pulp from the dilution tank to the screen.

In a pulp mill with an associated paper machine the short fibers and fines could be used used for paper furnish, replacing refined fibers. The short fiber fraction should have a length weighted average fiber length of 1.5 mm or less. The short fibers and fines need not be refined in order to be used. The fines content is at least 12% by weight.

Those skilled in the art will recognize that the present invention is capable of many modifications and variations without departing from the scope thereof. Accordingly, the detailed description set forth above is meant to be illustrative only and is not intended to limit, in any manner, the scope of the invention as set forth in the appended claims. 

1. Fractionated cellulosic fibers having a length weighted average fiber length of at least 2.4 mm and less than 2% by weight of fines having an average length or 0.2 mm or less.
 2. The fibers of claim 1 in which the fractionated cellulosic fibers have a length weighted average fiber length of at least 3.5 mm.
 3. The fibers of claim 1 in which the fractionated cellulosic fibers have a length weighted average fiber length in the range of 2.4 mm to 3.8 mm. 